J. S. Afr. Inst. Min. Metal/., vol. 89, no. 3.Mar. 1989. pp. 81-87.

The distribution of boron in stainless steelsas revealed by a nuclear technique

by J.P. HOFFMAN* and A.S.M. DE JESUSt

SYNOPSISAdditions of boron have been found to increase the hot workability of stainless steels. Boron forms complex

chromium-iron borides, primarily at grain boundaries but also on delta-ferrite stringers. This paper describes howa nuclear technique based on the track-etch method was used to distinguish between borides and chromium carbideson grain boundaries and to investigate the distribution of boron in stainless steels. The method was found to besensitive for boron, and made it possible for boron precipitates to be identified on grain boundaries and interphaseboundaries.

SAMEV A TTINGToevoeging van boron by austenitiese vlekvryestaal verbeter die warmverwerkbaarheid. Boron kan egter komplekse

chroom-yster boriedes op korrelgrense sowel as in delta-ferriet stringe vorm. Hierdie referaat beskryf hoe auto-radiografie gebruik is om die voorkoms en verspreiding van boron te bepaal en te kan onderskei tussen borieden karbied presipitate op die korrelgrense en interfase grense. In die tegniek word spore, gelaat deur alfa-deeltjies,gebruik om die posisies van boron vas te stel. Die tegniek is sensitief en spesifiek vir boron.

IntroductionBoron is added to high-alloy austenitic stainless steels

such as AISI 310, AISI 316, and AISI 317 to improvethe hot workability. It also has a beneficial effect if addedto duplex ferritic-austenitic stainless steels such as AISI318 (UNS S31803).

Boron (up to 60 p.p.m.) does not affect the hot- orcold-rolling characteristics of AISI 201 and AISI 304steels. However, the presence of even 10 p.p.m. of boronalters the microstructure of annealed AISI 201. Whenconventional mill-processing parameters are used, it ismost difficult to obtain a microstructure free of grain-boundary boride phases!.

An addition of boron improves the low-temperatureductility of stainless steels but also lowers the nil-ductilitytemperature. The higher the boron content, the strongeris this effect. Keown2 has given a good description ofthese effects. Fig. 1 shows the decrease in nil-ductilitytemperature for type 316 from approximately 1350°C(curve A, 9p.p.m. of boron) to 1200°C (curve D,170 p.p.m.), although the low-temperature ductility,measured as percentage reduction in diameter, has in-creased. From Fig. 1 it is apparent that there is anoptimum amount of boron for both a high nil-ductilitytemperature and a good low-temperature ductility range(curves Band C, 40 to 90 p.p.m. of boron).

Boron is also used to increase the creep properties ofoxidation-resisting steels and stainless steels used atelevated temperatures3. The addition of about 50p.p.m.leads to an increase in the mean stress-to-rupture life bya factor of 3 or an increase in stress to failure in 10 000. Middelburg Steel & Alloys (Pty) Ltd, p.a. Box 133, Middelburg,

1050 Transvaal.t Atomic Energy Corporation of South Africa, Private Bag X256,

Pretoria 0001.@ The South African Institute of Mining and Metallurgy, 1989. SA

ISSN 0038-223X/$3.00 + 0.00. Paper received July 1987.

~ 70°-C11-0

~ 60

100

90

80

50

1000

3011000

(2552)

Fig. 1- The effect of boron additions on the hot ductility of type316 stainless steel as a function of temperature2 (R of D =

reduction of diameter)

hours of up to 25 per cent.It was found recently that boron has a beneficial effect

(normally in combination with some titanium-typically0,015 per cent) on controlling a surface defect known as

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1989 81

600°C 650°C 700°C

No boron Boron No boron Boron No boron Boron

171 191 108 128 66 82

T ABLE IEFFECT OF BORON ON THE MEAN VALUE OF STRESSTO RUPTURE

IN 10000 HOURS FOR TYPE 3163(N/mm2)

Stress to rupture, N/mm2

edge slivers. This is probably due to its effect on hot duc-tility and hot workability4 (Fig. 2). Boron strengthensthe grain boundaries or hinders the segregation of elementsthat cause embrittlement or hot shortness. It also reducesgrain-boundary diffusion.

Boron segregates strongly to grain boundaries inaustenitic stainless steels as a borocarbide, M23(CB)6'and one could expect it to modify the resistance to inter-granular corrosion. Moskowitz et al.5 found that,although boron in solid solution is beneficial, precipitatedboron may be detrimental. Boron should preferably bepresent as the boronitride. In molybdenum-bearing steels,M23(CB)6 could be a complex carboboride6, (Fe, Cr,Mo)23(C,B)6'

Goldschmide has shown that the solubility limit ofboron in an 18Cr-15Ni steel containing less thanl00p.p.m. of carbon is 95 p.p.m., and that the precipi-tating phase is the boride (Fe,Cr)2B, which may form aeutectic of low melting point with the gamma phase attemperatures between 1150 and 1225°C, depending onthe composition of the base metals and the segregation.

It is quite possible for boron to be picked up fromrefractories and from teeming or casting powders. After

~2:CD~ 11,35

~uUJu...UJCl

a:UJ>...J11)u...0

>-~0::UJ>UJ11)

7,57

3,.78

"B~0040. CppmJ

the melting-refining process, boron is retained if theoxygen potential is low and deoxidants (reducing agents)and inoculants have been added.

Fig. 3 shows a portion of the phase diagram for an18Cr-15Ni stainless steel. The solubility of boron 7 isabout 60 p.p.m. in austenitic steels at solution heat-treatment temperatures of 1O50°C. Since 304 and 316stainless steels are solution heat-treated above 1O40°C,it should be no problem for the boron to be brought intosolid solution. However, the segregation of boron towardsthe grain and interphase boundaries probably occursduring cooling (or aging) since borides are often visibleon grain boundaries.

I t is often difficult to optically distinguish betweenboron and carbide precipitates (sensitization), and thetrack-etch technique was therefore developed to assist inthe identification of the types of precipitates on grainboundaries.

The track-etch technique is based on the creation ofnarrow regions of intense damage caused by the passageof heavy ionizing particles through a dielectric, Le.through most insulating materials. What happens is that,when the ion is travelling through the dielectric, itprimarily interacts with the electrons belonging to theatoms or molecules within the dielectric. In this process,electrons are either excited to a higher energy level orejected from their parent atom, and then themselves actin the same manner, Le. they interact with other electrons.Thus, the ionizing particle alternately loses some of itsown electrons and captures electrons from the medium.This purely Coulomb interaction is responsible for theslowing down of the moving particles and eventually forthe damage (tracks) induced in the bombarded material.The damage tracks can be revealed and made visible

12'\./!:.

~.~c?'84.~~

0,0030

82 MARCH 1989

Fig. 2-The effect of boron content and retention time on the Incidence of slivers In type 316 stainless steel4

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

TABLE IICHEMICAL COMPOSITIONS OF STEELS INVESTIGATED

(All the results are given in percentages by mass)

Specimenmarked Heat Type C S P Mn Si Mo Cr Ni B N

84 MARCH 1989 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Fig. 4-Track-etch showing boronsegregated in interdendrltlc deltaferrite (cast structure). Specimen

625, x 170

Fig. 5-Potasslum hydroxide etchshowing Interdendrltlc delta ferrite(cast structure). Specimen 625, x

160

Fig. 6- Track-etch showing boronsegregated In delta-ferrite stringers(hot-rolled and annealed structure).

Specimen 76866, x 170

Fig. 7-Ammonium persulphateetch showing borlde precipitatesmainly In delta-ferrite stringers.

Specimen 76866, x 256

Fig. 8- Track-etch showing boronsegregation on grain boundaries.

Specimen 658, x 170

Fig. 9-Ammonium persulphateetch showing borides on grainboundaries. Specimen 658, x 256

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1989 85

the microstructures with the boron images obtained bythe track-etch method. It should be noted that they arenot necessarily of the same area or to the same magnifi-cation.

Of special interest is the continuous network of car-'bides shown in Fig. 10, as against the 'pockets' or discon-tinuous network ('necklace') of boride particles in Fig. 9.

DiscussionCameron and Morral9 found that the solubility of

boron is higher in austenite than in ferrite at the invarianttemperature, thus proving that the reaction is eutectoidin nature, i.e. 1'- FezB + a.

Figs. 6 and 7 show boron-rich precipitates in ferritestringers, and Figs. 8 and 9 show the boron concentratedmainly in the interdentritic delta-ferrite phase. All fourphotographs tend to prove that the solubility of boronin the alpha (or delta) phase is lower than in the gammaphase.

Helium may be formed by the transmutation of theloB isotope during irradiation in a reactor, and precipi-tates as small bubbles that 1?roducepremature interg!anu-lar failure and embrittlement of austenitic steels in high-temperature tensile and creep rupture tests10. Back-ground images are present, which are believed to becaused mainly by the alpha tracks from the 170(n,a) 14Creaction taking place in the cellulose-nitrate film itself orthe helium bubbles as described above, or both.

Additions of boron to stainless steels occasionally resultin the appearance of a grain-boundary precipitate in theannealed structure of the finished product. This precipi-tate appears when the steel cools too slowly from theannealing temperature. A water quench is thus necessaryif the product is heavy section plate. In type 316, themolybdenum retards the precipitation of borides. Borondoes not appear on the grain boundaries at quenchingrates greater than some critical value between 50 and500°C per second6. Thomas and Henryll have indicatedthat solubility decreases rapidly with temperature andbecomes negligible below about 900°c.

In Huey tests (with boiling 65 per cent nitric acid) con-ducted for 164 hours1z, the progressive penetration

Fig. 10-Ammonlum persulphateetch showing true sensltizatlon, I.e.chromium carbides on grain bound-aries. Specimen 82-232 after a sen-sitization heat treatment, x 256

averaged 0,01 mm per month (standard 0,015 mm permonth). Thus, it appears1z that the boride type of grain-boundary precipitate does not adversely affect the inter-granular corrosion resistance of boron-containing 304.

Tests with 10 per cent oxalic acid (ASTM A-262-A)were used in the present work in the determination of theresistance to intergranular corrosion of all the steelsamples showing boron precipitation on the grain bound-aries. In all instances, the steel passed the test. It can thusbe concluded that the resistance to intergranular corro-sion of an austenitic stainless steel is not adversely af-fected by boron contents of 40p.p.m. or less.

Conclusions(1) The main advantage ofthe track-etch method for the

detection and, particularly, imaging of boron are thatit is virtually insensitive to beta, gamma, or protonradiation; it is specific for boron; and it lends itselfto the mapping of the boron distribution in a metal-lographic section.

.(2) It is possible to distinguish between boron precipita-

tion and the precipitation of any other element.(3) Boron precipitates preferentially on grain boundaries

and interphase (alpha-gamma) boundaries, or both,in hot-rolled plates.

(4) In the cast structure, boron segregates towards theinterdendritic delta-ferrite phase during cooling of theslab.

(5) The solubility of boron in Fe-Ni-Cr alloys is low7.(6) Boron contents of more than 90 p.p.m. may intro-

duce hot shortness in 304 and 316 steelsz.(7) Intergranular corrosion in 304 and 316 steels is not

impaired by boron contents of up to 40 p.p.m.

AcknowledgementsThe authors thank the Management of Middelburg

Steel & Alloys (Pty) Ltd and The Atomic Energy Cor-poration of South Africa for permission to publish thepaper.

86 MARCH 1989 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY